[stubs] Optimize LoadGlobalViaContextStub and StoreGlobalViaContextStub.

src/arm64/macro-assembler-arm64.h

Index: src/arm64/macro-assembler-arm64.h
diff --git a/src/arm64/macro-assembler-arm64.h b/src/arm64/macro-assembler-arm64.h
index f9aa8aa8e2b2b95419179af2d1596a75ac337d0b..135b9887ebee0b92617b8141f4c60ba3d090b9cb 100644
--- a/src/arm64/macro-assembler-arm64.h
+++ b/src/arm64/macro-assembler-arm64.h
@@ -1769,7 +1769,7 @@ class MacroAssembler : public Assembler {
   // |object| is the object being stored into, |value| is the object being
   // stored.  value and scratch registers are clobbered by the operation.
   // The offset is the offset from the start of the object, not the offset from
-  // the tagged HeapObject pointer.  For use with FieldOperand(reg, off).
+  // the tagged HeapObject pointer.  For use with FieldMemOperand(reg, off).
   void RecordWriteField(
       Register object,
       int offset,
@@ -2233,7 +2233,7 @@ class UseScratchRegisterScope {
 };
 
 
-inline MemOperand ContextMemOperand(Register context, int index) {
+inline MemOperand ContextMemOperand(Register context, int index = 0) {
   return MemOperand(context, Context::SlotOffset(index));
 }